I have a 414 small block in a 69 barracuda that I've recently got running. It has a new Carter mechanical fuel pump, the race version I don't remember the number, new 3/8 sender, new 3/8 lines from the tank, a holley 850 DP with a fuel pressure gauge at the inlet to the carb. I do not have a return line or pressure regulator. At idle, on initial startup the gauge will read 8psi, however after some run time it will drop to 5psi or sometimes even lower, I've seen less than 3psi at idle. I don't know what it is doing when driving, but reving the engine does not seem to change the pressure. The engine does not seem to be running out of fuel, but I've not driven it very much yet. Anyone have any ideas what could be causing the low fuel pressure?

Check all the rubber hoses on the fuel line from the fuel pump to the trunk.

I suspect a loose or dry rotted hose allowing air to enter the system. Easier to suck air than it is to suck fuel. You may not even see traces of fuel leakage since it is under vacuum.

otherwise I would check the pickup sock in the tank.

Just covering all the bases...What kind of gauge do you have ??? A cheapie or a good quality liquid filled can make a big difference in the accuracy of the reading...Been there done that....

Since the gauge is in the engine compartment, it may read lower the hotter it gets, especially if it is liquid filled. Try mounting the gauge temporarily outside the engine compartment and see if the reading still drops.

been there- done that. I agree with the statement above. The Liquid filled guage is the reason. Now, whether the fuel is too hot or the environemnt or a combination thats the question.

The problem was the liquid filled fuel pressure gauge. I replaced the gauge with another that was not liquid filled and now have good fuel pressure all the time, The non filled gauge does bounce around a lot, but it stays in the 7 to 9 psi range. It is much cooler here today than it has been, so we'll see if the problem returns in some form after the heat returns.